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Traditional diagnostic methods for tuberculosis, which include the skin prick test and whole-blood tests called interferon-gamma release assays, can’t distinguish patients with active tuberculosis from those who are no longer sick or have been vaccinated against the disease; doctors have been known to miss cases of the infectious bacterial disease in patients with HIV. Another common way to test for tuberculosis is to look for the disease-causing bacterium in sputum samples — a mixture of saliva and mucus — coughed up by patients. But researchers said that sometimes it’s hard for people to produce sputum on demand.

Now, researchers have identified a gene expression "signature" that can distinguish active tuberculosis from latent tuberculosis and other diseases: the Khatri blood test. They confirmed the diagnostic power of the test after administering it to 400 human samples previously collected for 11 different data sets.

The test, developed in the Khatri lab, removes the need to collect sputum because it requires an ordinary blood sample. According to the study, it’s capable of identifying tuberculosis infection even if the individual also has HIV. It can also diagnose all strains of the disease, even if it has evolved resistance to antibiotic drugs, researchers said. And it won’t give a positive response if someone only has a latent version of the disease or have received a tuberculosis vaccine.

"If the test comes up negative, it’s right 99 percent of the time. That is, of 100 patients who test negative with the Khatri test, 99 do not have active [tuberculosis]," researchers said.

These findings fill a need identified by World Health Organization (WHO) for better diagnostic tests for tuberculosis. In 2014, the organization called for a test that would give a positive result at least 66 percent of the time when a child has an active case of the bacterial disease. The Khatri test exceeds that, according to researchers,w who claim it’s 86 percent sensitive in children.

What’s more, the Khatri test not only accurately diagnoses people with active tuberculosis, but it also has potential to monitor patients to see how well they are responding to different treatments. Some doctors use the sputum test to monitor a patient's response to treatment but this method is rendered almost useless when patients can’t cough up an adequate amount of sputum. Also, researchers said, "As people start to get better, they can’t produce sputum for the test."

"One-third of the world’s population is currently infected with [tuberculosis]. Even if only 10 percent of them get active [tuberculosis], that’s still 3 percent of the world’s population — 240 million people," said Dr. Purvesh Khatri, assistant professor of medicine and senior author of the paper, in a statement.

Khatri and his team are now working to develop the test for widespread use, both to diagnose tuberculosis in patients and to monitor recovery in clinical trials, which will hopefully allow for more rapid development of better and cheaper treatment methods.